1. Field of the Invention
The present invention is directed to an arc welding apparatus. More particularly, the present invention is directed to an improved arc welding apparatus wherein physical contact of a welding electrode with a work surface is mechanically sensed, and the electrode is carried to a predetermined distance away from the work surface after the physical contact is made.
2. Brief Description of the Prior Art
Arc welding apparatus are well known in the prior art. As is well appreciated by those skilled in the art, during a welding operation the welding electrode must be at a predetermined distance from the work surface. This distance is usually determined principally on the basis of the nature of the electrode, the workpiece, the intensity of the electric arc employed for the welding, and the nature of the desired weld.
In order to position the electrode at the desired predetermined distance from the work surface (set in advance by an operator), welding apparatus of the prior art usually employ a lead screw actuated mounting assembly or carriage for the electrode. The lead screw is rotated by an electric motor and, depending on the direction of rotation of the motor, a threaded sleeve mounted to the lead screw carries the electrode towards or away from the work surface.
Because it is important to establish a reference point regarding the location of the electrode relative to the workpiece, the electrical resistance or impedance between the electrode and the workpiece is monitored in the prior art. The electrode is considered to be at zero distance from the workpiece when the monitored resistance or impedance falls below a predetermined threshold value. When this occurs, an electronic control unit causes the electric motor to reverse to carry the electrode away from the workpiece to a predetermined distance. As is known to those skilled in the art, the welding arc is usually initiated when the electrode is disposed at the predetermined distance from the work surface.
Although ideally, the above-summarized prior art welding apparatus should work reliably, in practice its operation is far from optimal. The principal cause for errors and unreliability of operation lies in the fact that a relatively large electric resistance or impedance may exist between the electrode and the workpiece in spite of the fact that the electrode physically touches the workpiece. For example, a corroded, rusty, or otherwise coated or unclean metal surface of the workpiece may offer an electrical resistance or impedance which is significantly higher than the threshold value. In this case the prior art welding apparatus does not "sense" that physical contact has been made between the electrode and the workpiece, and the motor driven lead screw assembly may continue to attempt to drive the electrode further into the workpiece. At the least, in the situation described above, the ability of the prior art welding apparatus for sensing contact between the electrode and the workpiece is seriously impaired.
Alternatively, in some welding apparatus of the prior art a welding power supply of low resistance is already connected to the electrode and the workpiece. In such a case the measured resistance or impedance between the electrode and the workpiece may fall below the threshold value even though the electrode does not physically contact the workpiece.
Accordingly, there is a need in the prior art for a welding apparatus having improved ability to sense the physical contact between the electrode and the workpiece. The present invention provides such an improved welding apparatus.